1. Field of the Invention
The present invention relates to an electrolyte solution for a secondary battery and a secondary battery using the electrolyte solution.
2. Description of the Related Art
A non-aqueous electrolyte lithium ion secondary battery or a non-aqueous electrolyte lithium secondary battery having an anode made of a carbon material, an oxide, a lithium alloy or metallic lithium is attracting attentions as electric power sources for cell phones or notebook computers, since the use of such battery provides higher energy density.
It is known that a film, which is referred to as surface film, protection film, solid electrolyte interface (SEI) or coating layer (hereinafter generically referred to as surface film), is formed on the surface of the anode of such type of the secondary battery. Since the surface film affects the efficiency of electrical charging and discharging, the cycle life and the safety of the secondary battery, it is known that the controlling the formation of the surface film is essential for achieving higher performances of the anode. When the carbon materials or the oxides are employed for the anode, the irreversible capacity of these materials must be reduced. When the metallic lithium anode or the lithium alloy anode is employed, the problems of lower efficiency of electrical charging and discharging or the problems of lower safety due to the formation of the dendrite crystal (tree branch-shaped crystal) must be solved.
Various methods have been proposed for the purpose of solving these problems. For example, it was proposed that the formation of the dendrite can be inhibited by forming a coating layer comprising lithium fluoride or the like on the metallic lithium substrate or the lithium alloy surface via the chemical reaction.
In JP-A-H7-302,617 is disclosed a technique of coating the surface of the lithium anode with the lithium fluoride film by exposing the lithium anode to an electrolyte solution containing hydrofluoric acid thereby reacting the anode surface with hydrofluoric acid. Hydrofluoric acid is generated by a reaction between LiPF6 and a slight amount of water. Meanwhile, a surface film of native lithium hydroxide or native lithium oxide is formed on the lithium anode surface via natural oxidation in the atmospheric condition. The surface film of the lithium fluoride, in turn, is formed on the anode surface by reacting the native oxide or hydroxide film and the hydrofluoric acid. However, since the lithium fluoride film is obtainable via the reaction between the anode interface and the solution, it is difficult to obtain uniform lithium fluoride film, as the surface film is easily contaminated with the reaction byproducts generated during the reaction. Further, when the surface film of native lithium hydroxide or native lithium oxide is not uniformly formed, the surface film may partially include bare lithium surfaces. In such case, uniform thin film of lithium fluoride is not obtainable, and further serious safety problems of possibly reacting the bare lithium surface with water or hydrogen fluoride may be caused. In addition, when the reaction is insufficient and not completed, unwanted other products than fluorides may remain in the surface film, thereby possibly causing harmful effects such as deterioration of ion conductivity. In addition, in the method of forming the fluoride layer by employing the chemical reaction occurred on the interface, the availability of fluorides and electrolyte solutions is strictly limited, thereby making it difficult to form the stable surface film with higher process yield.
In JP-A-H8-250,108 is disclosed a technique of forming a surface film of lithium fluoride on the anode surface by causing a reaction between a gaseous mixture of argon and hydrogen fluoride and an aluminum-lithium alloy. However, when a surface film of other material, in particular a surface film of a plurality of chemical compounds, already exists on the metallic lithium surface before forming the surface film of lithium fluoride thereon, reaction tends to proceeds non-uniformly, so that it is difficult to form a uniform film of lithium fluoride. In this reason, it is difficult to obtain a lithium secondary battery having sufficient cycle properties.
In JP-A-H11-288,706 is disclosed a technique of forming a surface coating structure comprising a material having rock-salt structure as a main component on a surface of a lithium sheet having an uniform crystal structure, i.e., [100] crystalline plane is preferentially oriented. It is also disclosed that this technique provides uniform deposition-dissolution reaction thereof, that is, uniform charging and discharging of the battery, so that the dendrite deposition is inhibited to improve the cycle life of the battery. It is further disclosed that the materials for the surface film preferably include halides of lithium, and more preferably include a solid solution of LiF and at least one selected from the group consisting of LiCl, LiBr and LiI. More specifically, an anode for non-aqueous electrolyte battery is manufactured by dipping a lithium sheet having preferentially oriented [100] crystalline plane and being manufactured via a metal pressuring (flat rolling) process into an electrolyte solution containing fluorine molecule or fluorine ion and at least one selected from the group consisting chlorine molecule or chlorine ion, bromine molecule or bromine ion and iodine molecule or iodine ion, in order to form the solid solution coating of LiF and at least one selected from the group consisting of LiCl, LiBr and LiI. This technique cannot sufficiently prevent the formation of the dendrite deposition, since the lithium sheet used in this technique is manufactured via flat rolling process, in which the lithium sheet may be exposed to the air atmosphere so that the native surface coatings derived by moisture and the like may easily be formed partially on the surface of the lithium sheet during the process, thereby non-uniformly distributing the active areas across the surface of the sheet, and being difficult to form the targeted stable surface film.
Further, NAOI et al. disclose in the 68th Annual Meeting of The Electrochemical Society of Japan (September 2000, at CHIBA INSTITUTE OF TECHNOLOGY, Number of presentation; 2A24), and The 41st Battery Symposium in Japan (November 2000, at NAGOYA INTERNATIONAL CONFERENCE HALL, Number of presentation; 1E03) that the influence of the complex of lanthanoid transition metals such as europium or the like and imide anion to the metallic lithium anode. It was presented in these presentation that an electrolyte solution is prepared by dissolving a lithium salt of LiN(C2F5SO2)2 into a mixed solvent of propylene carbonate or ethylene carbonate and 1,2-diethoxymethane, and further adding an additive of Eu(CF3SO3)3, and the surface film of Eu[(C2F5SO2)2]2 complex is formed on the Li metal surface that is dipped in this electrolyte solution. This method is effective in improving the cycle life in a certain level, but not sufficient. In addition, this method requires relatively expensive lithium imide salt such as LiN(C2F5SO2)2. Other Lithium imide salt cannot be employed for this method, since the complex of transition metal and imide anion is not formed when adding the complex of other lithium salt (e.g., typically LiPF6) transition metal and CF3SO3−F3S ion, and thus the cycle properties can not be improved. Further, the use of lithium imide salts as the electrolyte solution provides higher resistance of the electrolyte solution than the use of LiPF6 and the like, thereby causing a problem of increasing the internal resistance of the battery.
Further, it was reported that the use of carbon materials such as graphite or amorphous carbon that is capable of storing and emitting lithium ion for the anode improves the electrical capacity and the efficiency of charging and discharging.
In JP-A-H5-234,583, an anode comprising a carbon material that is coated with aluminum is proposed. This configuration inhibits reductional decomposition of solvent molecular that solvates with lithium ion, so that the degradation of the cycle life can be prevented. However, in this method, aluminum reacts with a slight amount of water, thereby causing a problem of rapidly decreasing the electrical capacity of the battery when cycles are repeated.
In JP-A-H5-275,077, an anode comprising a carbon material, a surface of which is coated with a thin film of lithium ion-conductive solid electrolyte. This configuration inhibits decomposition of the solvent that is occurred due to the use of the carbon material, and in particular, this configuration can provide a lithium ion secondary battery that is capable of using propylene carbonate. However, cracks are generated in the solid electrolyte due to the stress variation when lithium ion is inserted and desorbed, and the cracks lead to the deterioration of the properties thereof. Further, the quality of the solid electrolyte is not uniform due to the crystalline defects contained therein, and the non-uniformity of the quality leads to non-uniform reaction across the surface of the anode, thereby deteriorating the cycle life.
In JP-A-2000-3,724 is disclosed a secondary battery comprising: an anode comprising a material including graphite; an electrolyte solution containing a main component of cyclic carbonate and linear carbonate, and the electrolyte solution additionally containing 0.1% wt. to 4% wt. of 1,3-propanesultone and/or 1,4-butanesultone. It is considered that since 1,3-propanesultone and 1,4-propanesultone contribute the formation of the passivating coating on the carbon material surface, the active and highly crystallized carbon material such as natural graphite or artificial graphite is coated with this type of passivating coating to provide an advantageous effect of inhibiting the degradation of the electrolyte solution without deteriorating the ordinal chemical reaction for the battery. However, this method does not provide sufficient coating effect, such that the electrical charge generated by decomposition of solvent molecule or anion appears as an irreversible electrical capacity component, thereby decreasing the initial charge-discharge efficiency. Further, the resistance of thus formed coating is high, and in particular, the rate of the increase of the resistance with time or during storing in high temperature is considerably large.
As described above, the techniques disclosed in the prior art documents cannot provide sufficient improvement in the battery properties, and in particular, cannot provide sufficient effect on improving the charge-discharge efficiency and on preventing the increase of the resistance during storing. Thus, the following problems arise in the prior art.
The surface film formed on the anode surface closely relates to charge-discharge efficiency, cycle life and/or safety in relation to the characteristics of the film. However, there is no established method of continuously controlling the characteristics of the surface film in longer term. For example, when a surface film of lithium halides or amorphous or vitreous oxides is formed on a layer of lithium or alloys thereof, some effect of inhibiting the dendrite deposition appears in a certain level only in the initial operation. However, in the repeated operation, the surface film becomes degraded and the function of the protective film is impaired. It is considered that the reason of the degradation is the generation of the internal stress in these layers and on the interfaces therebetween, since the volume of the layers comprising lithium and lithium alloy varies due to the absorption and the emission of lithium, and on the contrary the volume of the coating thereon comprising lithium halides and so on does not substantially varies. The generation of the internal stress causes the breakage in a part of the surface film of, in particular lithium halides, thereby deteriorating the function of inhibiting dendrite growth.
Further, in relation to the carbon materials such as graphite, the surface coating cannot provide sufficient coating effect, the electrical charge generated by the decomposition of solvent molecule or anion appears as an irreversible electrical capacity component, thereby decreasing the initial charge-discharge efficiency. The compositions, crystalline condition, stability and so on of the formed film are considerably influential in the efficiency, the cycle life, the resistance and the increase of the resistance of the resultant battery. Further, a slight amount of moisture contained in the graphite or amorphous carbon anode promotes the decomposition of the solvent of the electrolyte solution. Thus, the elimination of the water molecule should also be carried out when the graphite or amorphous carbon anode is employed.
As described above, the coating formed on the anode surface closely relates to charge-discharge efficiency, cycle life and/or safety in relation to the characteristics of the film. However, there is no established method of continuously controlling the characteristics of the surface film in longer term, and it has been desired to develop an electrolyte solution, which is stable with the material of the anode and contributes to the sufficient charge-discharge efficiency.
In view of the above situation, it is an object of the present invention to provide a technology for inhibiting the decomposition of the solvent contained in the electrolyte solution for the secondary battery. It is also another object of the present invention to provide a technology for improving the cycle life of the secondary battery. It is yet another object of the present invention to provide a technology of improving the storing characteristics thereof such as the inhibition of the increase of the resistance of the secondary battery, retention rate of the electrical capacity of the secondary battery and so on.